Polyester films containing polyethylene terephthalate, polyethylene-2,6-naphthalate or the like are excellent in mechanical properties, heat resistance, dimensional stability, chemical resistance, cost-effectiveness and the like, and thus are used in many applications based on their performance. One of such polyester films is a thermal transfer ribbon. The thermal transfer recording method is used in fields such as FAX and barcode printing, because the method is excellent in cost-effectiveness, maintainability, operability and the like. In recent years, use of color thermal transfer inks has additionally imparted characteristics such as high definition and high image quality, and thermal transfer ribbons are also used in color thermal transfer printers and the like.
These thermal transfer methods are methods of superimposing, on an image receiving sheet, a thermal transfer ink ribbon including a polyester film and a thermal transfer layer that contains a coloring material such as a pigment or a dye and a binder such as wax provided on the polyester film, and applying heat using a thermal head from the back side of the thermal transfer ink ribbon to melt and fuse the thermal transfer layer onto the image receiving sheet, thereby forming an image on the image receiving sheet.
As for the thermal transfer ink ribbon, thinning of the polyester film is desired so that the transfer can be easily performed with less energy from the viewpoint of space saving of the ribbon and energy saving. A thin polyester film, however, has a problem that the polyester film is locally heated by the heat of the thermal head and perforated. In view of the problem, attempts have been made in order to improve the local heat resistance to alloy a highly heat-resistant resin to a resin that constitutes a polyester film (Patent Document 1), or to coat a polyester film surface (Patent Document 2).